Computer systems can accelerate database performance by utilizing a cache memory for buffering data transfers between the computer system and stable storage such as a hard disk drive. Cache memory has considerably better data access and data transfer characteristics than disks drives. Presumably, more frequently used data is more likely to be stored in cache memory. Also, data transfers between cache memory and disk can further be optimized by resequencing disk transfers to minimize positional delays inherent in a moving head disk drive. However, cache memory is volatile, that is, with the power removed, the data contents are removed.
An essential feature for a volatile cache memory is a suitable amount of random access memory (RAM) connected to a primary power supply. The RAM components are typically mounted on a printed circuit board (PCB) along with a backup power supply to power the volatile RAM during a failure in the primary power supply. By mounting the backup power supply on the PCB along with the RAM components the PCB can be removed from the system to facilitate servicing. Data retained in cache memory by a backup power supply can be subsequently written to stable storage when primary power is restored, thus maintaining the integrity of the database.
Failures in the primary power supply, such as voltage spikes, current dips or total power loss are typically sensed by electronic circuitry coupled to the primary power supply. This circuitry automatically disables the primary power supply when such failures are detected and almost simultaneously enables the backup power supply.
Since the lifetime of a cache memory can be as long as ten years, rechargeable batteries are generally used as the backup power supply. Traditional backup power supplies use rechargeable nickel cadmium batteries suitable for mounting on a PCB. A nickel cadmium battery can provide backup power from one to three months. Therefore, for long term backup power, rechargeable backup power supplies generally include multiple hierarchically coupled batteries to extend the useful backup time to about a year. However multiple rechargeable batteries increase cost and circuit space for a RAM cache circuit board. For example, to provide backup power to 1 megabytes of RAM for a year would require at least 6 nickel cadmium batteries consuming approximately 90 sq. cm of circuit board space.
Therefore, it is desirable that a volatile cache memory be constructed with a low cost compact and long life backup power supply.